1. Field of the Invention
The present invention relates to a process for preventing an occurrence of a transmuted layer (e.g., an oxidized layer or decarburized layer) in the surface of a hole defined in an forged article or the like which hole is difficult to eliminate the transmuted layer produced when a steel of the forged article is heated. It relates more particularly to a process which prevents or reduces an occurrence of an oxidized layer or decarburized layer by filling the hole with an inert gas, e.g., argon gas.
2. Description of the Prior Art
Hitherto, a hammer or press has performed an forging. A heating temperature of forging is 1,000.degree.-1,200.degree. C., a higher heating temperature more facilitating the forging. A normalizing temperature is about 900.degree. C. An annealing temperature is 650.degree.-850.degree. C. In addition, in order to prevent the occurrence of the transmuted layer in the surface of a hole defined in a holed forged article, copper plating or various coatings have been applied to the surface. However, copper plating can be effective only up to about 950.degree. C. and the coatings are difficult to eliminate after heating.
Herein, decarburization is a phenomenon in which carbon contained in steel is oxidized and released as a gas to the outside of the steel when the steel is heated in the atmosphere, so that a carbon concentration in a surface layer of the steel is reduced thereby to reduce a hardness of the surface layer. The reduction in the hardness reduces a fatigue limit of the steel. Herein, an oxidized layer means a zone of the steel in which the steel itself has been oxidized to be transmuted and which can be flaked from the surface of the steel in the form of scale.
In addition, a process of heating the holed forged article with an inert gas flowed has also prevalied. However, the process requires a larger amount of the inert gas, which is very costly. The effect of the flowed inert gas has been made unstable by the mixing of air.